vokera linea 28 hot water problem?

Similar problem with my Vokera plus combi. Works ok on CH, but on HW the gas constantly shuts off creating fluctuations in the hot water temp. The cycle is approx, gas on for 40secs, off for 20 secs repeating.
CH is fine when running alone.
If the HW is turned on whilst CH is running, a similar on of cycle occurs.
Very strange as my surname is also Watters. Help please

Steve Watters
September 2008